Hydraulic energy-conversion devices have been in wide use for decades. These are adapted to convert rotary mechanical torque to fluid pressure, and vice-versa. One type of this general class of machine utilizes an eccentric, cam, or crank on the rotatable shaft to induce reciprocating movement of a piston, and another type has a rotor driven by the shaft in an eccentric chamber, with moveable vanes on the rotor controlling the fluid as the shaft rotates. Other types referred to as centrifugal and gear devices are also common, but these are of no interest in connection with the present invention. All of these types have been subject to such intense development that general design details from all manufacturers of each type are closely similar. Performance criteria for comparing these devices include pressure and rotational speed capability, flow volume with respect to overall size, minimum alteration of performance from wear, and relative freedom from vibrations due to mechanical imbalance or pressure pulsations.